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Old 03-14-2005, 01:01 AM   #1
Kardon
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Question Sohc/dohc

Is there any performance difference?
Would a 250hp DOHC perform the same as a 500hp SOHC? Or is there no ratio?
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Old 03-14-2005, 01:14 AM   #2
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first of all do you know the difference between dohc and sohc?
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Old 03-14-2005, 01:19 AM   #3
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Not really, I thought DOHC was the standard, then I was browsing and saw that the Mustang is a SOHC. So I'm not sure of the difference.
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Old 03-14-2005, 01:34 AM   #4
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SOHC = single overhead cam (1 cam)
DOHC = Dual Overhead Cam (2 cams)

sohc = less valves = get air in/out semi-fast
dohc = more valves = get air in/out faster,

car companies generally use SOHC to make cars cheaper.
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Old 03-14-2005, 02:05 AM   #5
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Quote:
Originally Posted by Godlaus
SOHC = single overhead cam (1 cam)
DOHC = Dual Overhead Cam (2 cams)

sohc = less valves = get air in/out semi-fast
dohc = more valves = get air in/out faster,

car companies generally use SOHC to make cars cheaper.
Sooo... A SOHC engine with a four valve head has less valves than a DOHC engine with a two or three valve head? How exactly does that work?

Think, type, submit.
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Old 03-14-2005, 02:11 AM   #6
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DOHC and SOHC are generally just for timing... my DX civic is SOHC, and it has just as many valves as an SI DOHC civic... you just get a little more controll over timing with DOHC than you do with SOHC
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Old 03-14-2005, 02:13 AM   #7
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Quote:
Originally Posted by mazda6man
DOHC and SOHC are generally just for timing... my DX civic is SOHC, and it has just as many valves as an SI DOHC civic... you just get a little more controll over timing with DOHC than you do with SOHC
And how exactly is it that you "get a little more controll over timing with DOHC than you do with SOHC"? I think a factual explanation of the alleged principle involved is in order here.
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Old 03-14-2005, 02:18 AM   #8
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you can turn one cam without it affecting the others position... with a single cam, you turn it, and both intake and exhaust is affected... so say your off a feww degrees with say the intake timing cycle, but youre alright with the exhaust timing, you can mess with the intake to fix it and not bother with the exhaust
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Old 03-14-2005, 02:21 AM   #9
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Quote:
Originally Posted by vwhobo
Sooo... A SOHC engine with a four valve head has less valves than a DOHC engine with a two or three valve head? How exactly does that work?

Think, type, submit.

I was generalising, my bad. A Ford gt uses somehting like 32 valves off a pushrod, but was speaking in general terms.

Welcome back, by the way.
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Old 03-14-2005, 02:23 AM   #10
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Quote:
Originally Posted by mazda6man
you can turn one cam without it affecting the others position... with a single cam, you turn it, and both intake and exhaust is affected... so say your off a feww degrees with say the intake timing cycle, but youre alright with the exhaust timing, you can mess with the intake to fix it and not bother with the exhaust
That's a stretch at best. You're assuming that he's talking about a modified engine... As an engine would have to be to have adjustable timing gears/camshaft sprocket. Why can't we simply modify it by using a different cam? Same results, different approach and you "control the timing" in exactly the same manner.

Bottom line is you have no more "control of the timing" with a DOHC engine, you simply make the adjustmants in a different way.
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Old 03-14-2005, 06:53 AM   #11
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Hahahaha, you don't have more control over timing with a DOHC, why would you want to change the timing from one cam to another anyway... that would be pointless.

Every post in this thread has been WRONG except vwhobo's posts.

The difference between the two obviously is one has two cams when the other has one, both have same amount of valves.

Mustang only has 3 valves by the way so that's a bad example.

Okay, the reason that DOHC is better is the head design, SOHC is limited to the design of the combustion chamber, and is not able to create a very efficient chamber, because the cam sits between the exhaust and intake valves so it's pushing away. DOHC has a cam for intake and exhaust and is able to push the valves the other way (towards the inside of the combustion chamber) making it more efficient.

Hope that was explained well enough, I ain't no teacher.
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Old 03-14-2005, 12:44 PM   #12
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Quote:
Originally Posted by 88GrandPrixSE
Hahahaha, you don't have more control over timing with a DOHC, why would you want to change the timing from one cam to another anyway... that would be pointless.

Every post in this thread has been WRONG except vwhobo's posts.

The difference between the two obviously is one has two cams when the other has one, both have same amount of valves.

Mustang only has 3 valves by the way so that's a bad example.

Okay, the reason that DOHC is better is the head design, SOHC is limited to the design of the combustion chamber, and is not able to create a very efficient chamber, because the cam sits between the exhaust and intake valves so it's pushing away. DOHC has a cam for intake and exhaust and is able to push the valves the other way (towards the inside of the combustion chamber) making it more efficient.

Hope that was explained well enough, I ain't no teacher.
Now including yours as well. There are several reasons you might "want to change the timing from one cam to another". The most obvious being adjusting your powerband by varying the LSA.

BTW, your reasoning for why DOHC is better than SOHC is, in a word, wrong. While it has some merit in a theoretical kind of way, in the real world there are many ways manufacturers have found to overcome this minor nuisance
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Old 03-14-2005, 04:53 PM   #13
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a correction here. the mustang is not really an SOHC.

if it was an SOHC, then it would have the camshaft on top of the valve head. the mustang is an OHV type of engine, aka Pushrod.

OHV = OverHead Valves

in this case, the camshaft is located right on top of the crankshaft case, and it moves the valves by pushing these "sticks" that are called pushrods. quite simple.

also, not all mustangs have just 3 valves. also, the number of valves really doesnt have anything to do with the timing of the cams. they just affect the performance delivered by affecting how well the engine breaths. engines with fewer valves have even bigger sized valves in place.

also, an SOHC engine does NOT have a limit to the size of the cam or anything. in fact, i have never seen a camshaft thick enough to be compared to the width of a hotdog inside of a bun and napkin (not counting the displacement of the lobes, just the center shaft). in the newer VR6 engines, the intake camshaft controls the intake valves on the opposite side of the engine via some extended shafts that come out of the rocker arms, and vice versa with the exhaust camshaft. this can be done with a simple SOHC engine, in fact, it would be even easier. how? simple, cuz this is how pushrod engines literally work (differently, but they "push sticks to open the valves", in simple terms).
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Old 03-14-2005, 05:41 PM   #14
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Quote:
Originally Posted by Inygknok
a correction here. the mustang is not really an SOHC.

if it was an SOHC, then it would have the camshaft on top of the valve head. the mustang is an OHV type of engine, aka Pushrod.

A litle correction here... The Mustang hasn't had a pushrod engine since '95. The standard V8 Mustang since then has been the 4.6 liter SOHC (yes, overhead cam) engine, with the later Cobras getting a DOHC version.

Man, why is it that people can't even get the basics right?

Oh, and while technically, pushrod engines are OHV, so are SOHC and DOHC engines! In all cases, the valves are "above" the piston, i.e. overhead.

And Kardon, as to your original question,

Quote:
Is there any performance difference?
Would a 250hp DOHC perform the same as a 500hp SOHC?

Think about it... In the same car, would a 250 hp engine of ANY sort perform like a 500 hp version? If they did why would having more power in the same car be any better?

With the same gearing and same powerband, a 250 hp engine would perform like a 500 hp engine only if the car it was in was HALF the weight of the car that the 500 hp engine was in. Basic math. Has nothing to do with how the valves are actuated.
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Old 03-15-2005, 04:53 AM   #15
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Quote:
Originally Posted by vwhobo
BTW, your reasoning for why DOHC is better than SOHC is, in a word, wrong. While it has some merit in a theoretical kind of way, in the real world there are many ways manufacturers have found to overcome this minor nuisance

Actually, no they haven't. A DOHC engine will always make a bit more power than a SOHC equivalent. They're close, yes, but DOHC still does make more power. Might only be another 10hp on a 200hp engine, but still an improvement no matter how you look at it. You're much more limited to how you can design the heads of a SOHC over a DOHC.
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